1. Field of the Invention
The present invention relates to an image printer for creating a hard copy of a television image or an image produced by computer graphics or the like. More particularly, the invention relates to an apparatus for driving a thermal printing head in the image printer.
2. Description of the Prior Art
FIG. 1 is a block diagram schematically showing a conventional image printer. In this figure, an image signal is applied to an input terminal 101. Vertical and horizontal synchronizing signals are applied to another input terminal 102. The image signal is digitized by an A/D converter 103. Indicated by reference numeral 104 is a digital image memory. A density conversion unit 105 is provided to obtain various gradations corresponding to variation from white to black from the image data. A system control unit 107 includes a counter and gate circuits, and produces clock pulses for attaining synchronized operations, as well as other signals necessary for various circuits. A comparator 106 compares data C5 delivered from the density conversion unit 105 with an output C4 from the counter incorporated in the system control unit 107. A printer 108 comprises a thermal printing head and a mechanism that mainly consists of a means for moving paper on which the head will part.
An operation of this apparatus will be described below.
The image signal applied to the input terminal 101 consists of a luminance signal in the form of the NTSC television signals. Signal ranging from a reference black level to a reference white level is quantized into six bit digits, for example, by the A/D converter 103. Then, the quantized image data is successively written to the image memory 104 from an address previously specified. It is assumed that the number of the sampled data items of one horizontal scanning line is 640 and that one frame consists of 480 horizontal scanning lines. The capacity of the memory is 640(H).times.480(V).times.6 bits=1.8 Mbits. The image data for one frame stored in the memory 104 is read out in a given sequence and converted into 8-bit time axis data by the density conversion unit 105 so that printing may be performed on the paper with 6 bits in 64 gradations. The system control unit 107 includes a counter which counts the number 256 in the print time for one scanning line and expresses time in 8 bits. The output C4 from the counter is compared with the data C5 delivered from the density conversion unit 105 by the comparator 106. The print head of the printer 108 is activated or deactivated according to the output C6 from the comparator 106 to selectively print.
FIG. 2 shows the structure of the thermal printing head and an example of driving the head. The head comprises 640 resistance members, for example. If the 640 resistance members are simultaneously energized or deenergized, this causes undesired excessive electric power consumption. Therefore, the head is divided into several blocks which are successively energized for a printing operation. In this example, the head is divided into five blocks B1-B5 as shown in FIG. 2. Each block includes 128 resistance members. Six hundred forty (640) data for one scanning line are read out from the memory 104 during one counting operation for count data C4 of an 8-bit counter. These data are converted by the density conversion means 105 into 8-bit, time axis data which are successively compared with the data C4 by the comparator 106. At this time, a signal of low level is produced to deactivate the head in case of the relation being C4.gtoreq.C5. When C4&lt;C5, a signal of high level is produced to activate the head. These operations are successively carried out for all the data items. The output signal C6 from the comparator is supplied to a register in the head of the printer 108 together with the transferred clocks contained in a signal C7. These signals are temporarily stored in the register. Then, those signals which are contained in the signal C7 to activate the blocks of the head are successively made to go high during the periods T.sub.1, T.sub.2, T.sub.3, T.sub.4, T.sub.5, respectively, to print 1/256 of the N-th the scanning line. Thus, printing at count 0 specified by the count signal C4 is completed. Thereafter, image data items for the N-th scanning line are again read from the image memory 104 and successively converted into time axis data items C5 by the density conversion means 105. The output C5 from the conversion means 105 is compared with the updated count signal C4 that assumes value "1" to produce the signal C6 which successively activates or deactivates the resistance members. Then, the same operation is performed as in the case of count "0". As a result, printing at the count "1" specified by the signal C4 is completed. This process is repeated until count 255 is reached, in order to complete a printing operation for the N-th scanning line. The same process is carried out for the (N +1)th scanning line as in the case of the N-th scanning line. These operations are repeated for 480 scanning lines to create a hard copy of an image.
At this time, the mechanism of the printer 108 intermittently or continuously feeds the paper. The movement of the paper is so controlled that the aspect ratio is 3:4 that meets the standard of the NTSC television. The system control means 107 produces various signals including C1, C2, C3, C4, C7, C8 in response to both the synchronizing signal applied to the input terminal 102 and the clocks generated inside the control means 107. The clocks Cl are applied to the A/D converter. The control signal C2 consists of a signal indicating addresses and a read/write signal, and is applied to the image memory 104. The clocks C3 are necessary for the density conversion unit 105. The count signal C4 is delivered from the counter and indicates the elapse of time with 8 bits. The clocks C7 are needed for the operation of the head. The signal C8 is required for sequentially controlling the printing mechanism. A desired black-and-white hard copy of an image is produced in 64 gradations by the structure and the operations described above.
The above described conventional apparatus is disadvantageous in that the densities at picture elements which neighbor in the direction of the movement of the paper are different because of thermal hysteresis. In the case where a print is made at low density, the activated portions of the head are concentrated in forward portions for the print of one picture element. Therefore, the printed image looks rough. In any case, the print quality is poor.
Another conventional apparatus will be described in more detail with reference to FIG. 3 which is a block diagram showing a conventional apparatus for driving a thermal printing head. In FIG. 3, data C11 regarding an image or another form of data converted from the image data is applied to an input terminal 201. A signal C12 for switching between read mode and write mode is applied to another input terminal 202. A line buffer 206 comprises a digital memory for storing data for one line of an image. A write control circuit 203 produces write addresses and other data for the line buffer 206. A read control circuit 204 comprises a block address counter 204a and an address counter 204b, and produces read addresses and other data for the line buffer 206. A selector 205 selects either a write control signal C13 or a read control signal C14 as a control signal C15 applied to the line buffer 206. A time axis counter 207 determines the time for which a print is made. A comparator 208 compares the image data or data converted therefrom C18 which is delivered from the line buffer 206 in a desired sequence with a time axis data C19 delivered from the time axis counter 207. An output terminal 209 supplies data C20 to the thermal printing head of the thermal printer.
An operation of the above will be described with reference to FIGS. 2 and 3. It is now assumed that the image data C11 for one line which is applied to the input terminal 201 is 8-bit data. A write signal at high level is applied to the input terminal 202 for switching between read mode and write mode, in order to put the line buffer 206 into write mode. At this time, the control signal C15 fed to the line buffer 206 is produced as a write control signal C13 from the write control circuit 203 through the selector 205. Input data C11 for one line is stored in the line buffer 206 at addresses specified by the write control circuit 203. The control signal C15 includes address signals, an output enable signal, and other signals.
Then a read signal that is at low level is applied to the input terminal 202 for switching between read and write modes, in order to bring the line buffer 206 into read mode. Under this condition, the control signal C15 is produced delivered from the read control circuit 204 as a read control signal C14 through the selector 205. Data is read from the line buffer 206 in a desired sequence.
The time axis counter 207 counts 2.sup.8 =256. bits of data at given intervals. The comparator 208 compares the time axis data C19 delivered from the counter 207 with the output C18 from the line buffer 106. The output from the comparator 208 appears at the output terminal 209 as a data signal C20 that determines the time for which the head is activated.
Assuming that the block address counter 204a in the read control circuit 204 specifies the first block, the address counter 204b successively specifies addresses in the block. Thus, 128 bits of data concerning the first block are read out.
The data C18 read out is compared with "0" of the output data C19 from the time axis counter 207 by the comparator 208. When the value of the intensity data C18 is less than the amount of the count data C19, the head is deactivated. In this state, the data signal C20 is made to be low. When the value of the intensity data C18 is equal to or larger than the amount of the count data C19, the head is activated. In this state, the data signal C19 is caused to be high. The data signal C20 is delivered continuously from the output terminal 209 and transferred to a register that is provided within the head for the block B1. Then, the data is temporarily stored there.
These signals are used to activate and deactivate the first block B1 of the head of the thermal printer, in order to print, for 1/25 of printing time of one block. Subsequently, the time axis counter 207 counts input data forward until "1" is reached. In this state, a printing is performed using the first block B1 in the same manner as the foregoing.
Thus, the time axis counter 207 increments 256 times until the value the count output reaches "256", and 256 printing operations are made, thus completing the printing using the first block B1. Thereafter, the block address counter 204a specifies the second block B2. Prints are made with the second block B2 in the same manner as the aforementioned process. Similar prints are made with the third through fifth blocks B3-B5. As a result, one line of printing is completed. During this process, the first block B1, the second block B2, the third block B3, the fourth block B4, the fifth block B5 are activated for periods T1, T2, T3, T4, T5, respectively. Only one of the blocks B1-B5 is activated at a time.
It is assumed that it takes about 37.5 .mu.s to print 1/256 of a total printing period one picture element. In order to completely print one block, a period 256 times as long as that interval, i.e., about 9.6 ms, is required. To print one line, a period five times as long as the period, i.e., about 48 ms, is needed. Where one image is formed by 480 lines, it takes about 23 seconds to make a print of one full image.
The above described conventional apparatus for driving the thermal printer head is disadvantageous in that the head is not activated at least for a period 4/5 times the period taken to print one line, and the head is thus deactivated continuously for about 38 ms out of about 48 ms taken to print one line resulting in the blocks adjacent to the block which is presently used for printing being cool and a large temperature difference therebetween. Accordingly, the temperature of both ends of the block that is now used for printing increases at a rate lower than that of the central portion and so the print density at both ends of the black is lower than the density at the central portion of the black. In this way, the density becomes nonuniform within one block. This creates white stripes at the boundaries between neighboring blocks.